The 1986 Amendments to the United States Safe Drinking Water Act required the U.S. Environmental Protection Agency to implement regulations requiring disinfection of all public water supplies. The regulations also specified water quality criteria under which filtration of surface waters would be a requirement and listed Giardia Lamblia and viruses among a list of 83 contaminates that would be regulated in drinking water. Ozonation is one of many methods used for the purification of water. It is a technology substantially more effective than others. Ozone not only can kill bacteria and viruses, but also oxidize organics such as detergents, pesticides, herbicides, phenols and inorganics such as iron, manganese, organically bound heavy metals, cyanides, sulfides, and nitrates. Ozone has been used to treat ground and surface water in many European cities for years and also is becoming the industry standard for treating bottled water.
Ozone, also referred to as triatomic oxygen, is an unstable gas having life in water of minutes. Oxygen, which is normally bi-atomic, becomes ozone through the addition of a third unstable atom. Ozone, because of its instability, cannot be generated and stored for future use. It must be generated and used for treatment immediately. It is created by one of two generation methods: ultraviolet radiation or corona discharge. Of the two, corona discharge produces the substantially higher ozone concentration needed for the removal of complex impurities. Generated ozone is pumped into the water through an ozone diffuser, a stone of fine porosity, creating very small bubbles which rise slowly through the water. The slower the bubbles rise through the water, the greater the amount of ozone transferred to the water.
Most critically for water quality, ozonation does not add chemicals to the water as does chlorine, chlorine dioxide, permanganate, etc. As the ozone passes through the water, the third unstable atom detaches, attacks, and destroys impurities in the water. The residue in the water is pure oxygen, which quickly dissipates. Any excess ozone which is not needed for treatment reverts to simple oxygen.
Two main schemes for ozone reactions in water have been proposed (see J. Hoigne and H. Bader. 1975. Ozonation of Water: Role of Hydroxyl Radicals as Oxidizing Intermediates. Science, 190 (4216): 782-784). The first is direct oxidation, which is a selective oxidation of chemical compounds by the ozone molecule. These reactions are quite selective and can take minutes. In the absence of ozone decomposition, the ozone concentration can remain relatively constant over short time intervals. The second scheme relates to oxidation by intermediate radical species where the hydroxyl radical is believed to be the most important of the ozone decomposition products. The hydroxyl radical is highly reactive and has a life span of only a few microseconds in water. The mode of action of ozone on microorganisms is poorly understood. Some studies using bacteria suggested that ozone altered proteins and unsaturated boned of fatty acids in cell membrane, leading to cell lysis. Other studies have suggested that ozone may affect deoxyribonucleic acid (DNA) in the cell, causing cell inactivation. Virus inactivation was reported to be related to attack of protein capsid by ozone.
The ability of ozone to disinfect polluted water was recognized as early as 1886. Ozonation might have become universal for disinfection in water treatment except for the introduction of cheap chlorine gas. Recently, however, concern has been expressed about possible toxic effects of chlorine to human health. In search for alternatives to chlorine, ozone is being considered.
Ozone is toxic and dangerous to human, animal and plant life, including aquatic forms. Conventional bottled drinking water production systems consist of three major components: the ozone generator, ozone water mixer and offgas ozone destroyer. Although bottled drinking water is disinfected by ozonation, it is still quite possible that the bottled drinking water can be polluted by a secondary contamination occurring during sealing, transporting, storing and using the product. It is known that bacteria and viruses multiply fast, especially when bacteria and virus inhibiting substances are removed from bottled drinking water. Sometimes the level of bacteria and viruses in bottled drinking water actually may be higher than that of tap water if the bottled drinking water is stored for a long time.
Ideally, it is better to drink fresh, bacteria free, clean water. Therefore, an effective ozone water purifier suitable for household use is highly desirable. In an industrial bottled drinking water production system, the design of ozone water mixer and offgas ozone destroyer is relatively easy because there is no space limitation. However, for small ozone water purifiers suitable for household use, designing a compact device is not simple without sacrificing the quality of purified drinking water. In a thermal ozone destruction unit, the offgas is heated to a prescribed temperature, typically between 300.degree. C. and 350.degree. C. for a short period of time. Such a unit can not be employed in a compact ozone water purifier suitable for household use. Major components of household devices and appliances are often made of plastics for economic and other reasons, and these materials usually cannot withstand such high temperatures.
In ozonation systems disinfection efficiency depends on the dose of ozone injected in water, ozone and water mixing, ozone and water contact time and reaction rate between ozone and water impurities. High dose of ozone, homogeneous mixing, long contact time and fast reaction rate between ozone and water impurities are essential to achieve high disinfection efficiency.
U.S. Pat. No. 5,427,693 discloses an apparatus used for treating contaminated water. A 50 to 200 feet length of tubing is used to enhance the contact time. Obviously, using such a long tubing is not practical in household drinking water purification because of space limitations. U.S. Pat. No. 5,250,177 also uses a long tubing to enhance the contact time. However, use of tubing only will not achieve high disinfection unless very long tubing is used. U.S. Pat. No. 5,851,407 discloses an apparatus using ozone and hydrogen peroxide for water decontamination. One type of static mixer is disclosed. While the effectiveness of disinfection is allegedly enhanced, the contact time is not enough for drinking water purification. U.S. Pat. No. 5,766,488 discloses an apparatus in which the ozonizer serves as a static mixer. This apparatus is believed too complicated and costly to be used in households. U.S. Pat. No. 5,888,403 utilizes a complicated static mixer for ozone and water mixing. Each of the patents referenced above only emphasizes one of the factors which affect disinfection effectiveness. However, all the factors have to be optimized in order to achieve high disinfection.
The known prior art fails to teach or even suggest the combinations of structural elements and method steps disclosed and claimed herein which cooperate to provide a highly efficient, inexpensive approach for ozone treatment of water which is practical and suitable for household as well as other uses.